High speed wordline decoder for driving a long wordline

ABSTRACT

A method and apparatus for improving the performance of a memory wordline decoder is disclosed. A decoder latch is attached to an inverter which drives the wordline. Additionally, a voltage pump can supply operating voltage to the inverter to assist in overdriving the wordline. A voltage sink can also be coupled to the inverter which, in combination with the voltage pump, can be used to shift the output voltages used to turn the wordline on and off. A second inverter can also be added, and in such a case the transistors within the latch and the first inverter can be reduced in size, switching time, and power consumption.

FIELD OF THE INVENTION

The present invention relates to the field of memory address decoders,and particularly to address decoders for driving long wordlines of amemory device, for example, a flash memory device.

BACKGROUND OF THE INVENTION

To achieve high access speeds in memory arrays, including those of flashmemory devices, addressed wordlines must be driven as fast as possible.Fast wordline decoder devices for long wordlines tend to require complexlatch circuitry for properly driving the wordline. Such circuits drawconsiderable power. A less complex wordline decoder device which drawsless power and occupies less die area is therefore desirable.

BRIEF SUMMARY OF THE INVENTION

In one aspect, the invention provides a wordline decoder and operatingmethod having wordline decoding pass transistors, a latch for initiatingthe driving of a wordline in response to the pass transistors decoding awordline, and an output buffer responsive to a switching state of thelatch for driving the wordline. In an additional aspect of theinvention, the wordline decoder includes a voltage pump and voltage sinkfor supplying operating voltage to the buffer. Since the wordline isdivided by the output buffer, the buffer can be optimally designed fordriving the wordline, while the latch can be optimally designed forswitching speed.

These and other features and advantages of the invention will be moreclearly understood from the following detailed description provided inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a decoder driver incorporatingconventional latch circuit which directly drives a wordline;

FIG. 2 is a schematic diagram of a first embodiment of the decoderdriver of the present invention;

FIG. 3 is a block diagram of the present invention implemented within acomputer system; and

FIG. 4 is a schematic diagram of a second embodiment of the decoderdriver of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an address decoder driver which canquickly drive wordlines having substantial resistance and parasiticcapacitance due to wordline length and the number of devices attachedthereto. Additionally, the decoder driver output can shift the levels ofthe voltages presented at its inputs, which makes it useful across avariety of memory platforms.

A typical address decoder driver can drive a wordline by an output ofdecoder 101 which drives a latch formed by cross-coupled transistors104, 108, such as the latch 100 shown in FIG. 1. However, by themselvesthe cross-coupled transistors 104 and 108 within the latch 100 are notstrong wordline drivers. As the capacitance on the wordline 112increases, the transistor 108 needs to also increase enough to switchthe latch 100. However, if Vcc is below a certain limit, the transistor108 cannot switch the latch 100.

Accordingly, while adequate to drive shorter wordlines with lowercapacitance, the latch 100 has difficulty when driving wordlines havinga relatively large capacitive load, such as wordlines connected to 1K ormore memory cells. An improvement in the FIG. 1 circuit can be obtainedif the latch 100 is used with a voltage pump, which permits the latch todrive the wordline 112 with the output of the pump voltage. Voltagepumps are used to overdrive the wordline 112 with the pump voltage.However, modifying the prior art latch 100 of FIG. 1 with a voltagepump, set higher than Vcc, would still not reliably drive a wordlinewith a substantial amount of capacitance, and also increases complexityof the latch 100.

FIG. 2 shows an exemplary embodiment of a decoder driver circuit 200 ofthe present invention. The decoder 101 is shown as decoding a largestrectangle with an X4 designated in the upper right hand corner whichsymbolizes 1 of 16 memory banks. Within that rectangle is shown anotherrectangle with X8 also in the upper right hand corner, which designates1 of 256 memory blocks within that memory bank. The third, smallestrectangle labeled X8 in the upper right hand corner designates 1 of 256rows within that memory block. The decoder driver shown in FIG. 2 isused to select, by output of decoder 101, one of four memory banks,denoted by the selection rectangle X4 (201), one of two hundred andfifty-six memory blocks within a selected bank, denoted by selectionrectangle X8 (203), and one of two hundred and fifty-six rows within aselected memory block denoted by X8. The decoder 101 includes N-channelpass transistors N4, N5, and N6 which decode the information arriving onaddress lines XEN<3:0>, XB<3:0>, XA<7:0>, and XROW<7:0>. A specificwordline 228 is then selected using that address information. It shouldbe noted that the present invention is not limited to this specificmemory architecture, which is shown for illustrative purposes only.

By itself, the latch 236 can not sink the amount of current needed todrive long wordlines, due to the inherent resistance and capacitancepresent therein. Consequently, as shown in FIG. 2, the present inventionadds a higher strength output buffer formed as inverter 232, consistingof a p-channel transistor 220 and an n-channel transistor 224 to theoutput of the latch 236. In this way, the latch 236 drives the inverter232 which then drives the wordline 228, rather than driving the wordline228 directly. Consequently, the transistors within the inverter 232 canbe tailored to be larger and drive more current, depending on the typeof memory in which the decoder 200 is installed. Additionally, the latch236 of FIG. 2 can be tailored to have better switching propertiesbecause the burden of driving the wordline is shifted to the inverter232. The inverted stage 232 (FIG. 2) serves to isolate the capacitiveload present on the wordline 228 from the transistor 208. With theaddition of the inverter 232, the transistor 208 can be reduced in size,and thus the entire device 200 can be operated at a lower Vcc.

To further assist the driving of the wordline 228 a voltage pump VPXB212 may be used to provide an operating voltage to buffer 232. Thevoltage pump may also be used to supply operating power to latch 236.The voltage level of the voltage pump VPXB 212 is set higher than Vccand is attached to the source of the transistor 220 within the inverter232. Furthermore, a voltage sink VSXB 216 set at a negative voltagelower than ground can be used in conjunction with the voltage pump 212to achieve output voltage level shifting, so that the decoder 200 of thepresent invention can accommodate a wider variety of memory devicesdriving the wordline between the voltage of the pump 212 and sink 216.This is useful when working with high-voltage syncflash memory.

To minimize current loss during the switching process, it is desired toswitch the node NX (FIG. 2) as fast as possible. When the voltage at thenode NX descends toward ground, current is being drawn from VPXB voltagepump through the PMOS transistor 208. The function of the PMOStransistor 204 is to bring up the gate of the transistor 208 to thevoltage level of VPXB in order to shut off the transistor 208 as fast aspossible. In either case, up or down, it is desired to transition thenode NX as fast as possible. Selecting the sizes of the PMOS and NMOStransistors within the 236 is an important feature of the presentinvention. It is desired to have the transistors 204 and 208 to be smallas possible, saving area and reducing switching current. However, theinverter 232 still has to be large enough to drive a large capacitanceload on the word line 228.

A second embodiment of the buffer 200 is shown in FIG. 4, wherein asecond output stage 404 assists in driving the wordline 228. In caseswhere the capacitive load on the wordline 228 increases to 4K or morememory cells or gates, the inverted stage 232 becomes very large, whichin turn becomes a large load for the cross-coupled transistors 204 and208. Adding another inverted stage 404 reduces the size of the invertedstage 232, which in turn relieves pressure on the cross-coupledtransistors 204 and 208.

In the first embodiment, to support 1.8 volt supply voltage it wasnecessary that transistors 220 and 224 be large and thuspower-consuming. However, in the second embodiment shown in FIG. 4, itis not necessary that the transistors within the second output stage 404be as large and power-consuming. Thus, the circuit of FIG. 4 has betterswitching properties because the burden of driving the wordline isshifted to the second inverted stage 404. With the addition of thesecond inverted stage 404, the transistor 208 can be even furtherreduced in size, and the entire device 200 can be operated at even lowerVcc levels, including below 1.8 volts.

FIG. 3 illustrates an exemplary processing system 300 which may utilizean electronic device comprising a self-biasing buffer constructed inaccordance with the embodiments of the present invention disclosed abovein connection with FIGS. 1-2. The processing system 300 includes one ormore processors 301 coupled to a local bus 304. A memory controller 302and a primary bus bridge 303 are also coupled the local bus 304. Theprocessing system 300 may include multiple memory controllers 302 and/ormultiple primary bus bridges 303. The memory controller 302 and theprimary bus bridge 303 may be integrated as a single device 306.

The memory controller 302 is also coupled to one or more memory buses307. Each memory bus accepts memory components 308 which include atleast one decoder 200 of the present invention. The memory components308 may be a memory card or a memory module. Examples of memory modulesinclude flash memory modules or cards, single inline memory modules(SIMMs), and dual inline memory modules (DIMMs). The memory components308 may include one or more additional devices 309. For example, in aSIMM or DIMM, the additional device 309 might be a configuration memory,such as a serial presence detect (SPD) memory. The memory controller 302may also be coupled to a cache memory 305. The cache memory 305 may bethe only cache memory in the processing system. Alternatively, otherdevices, for example, processors 301 may also include cache memories,which may form a cache hierarchy with cache memory 305. If theprocessing system 300 include peripherals or controllers which are busmasters or which support direct memory access (DMA), the memorycontroller 302 may implement a cache coherency protocol. If the memorycontroller 302 is coupled to a plurality of memory buses 316, eachmemory bus 316 may be operated in parallel, or different address rangesmay be mapped to different memory buses 307.

The primary bus bridge 303 is coupled to at least one peripheral bus310. Various devices, such as peripherals or additional bus bridges maybe coupled to the peripheral bus 310. These devices may include astorage controller 311, an miscellaneous I/O device 314, a secondary busbridge 315, a multimedia processor 318, and an legacy device interface320. The primary bus bridge 303 may also coupled to one or more specialpurpose high speed ports 322. In a personal computer, for example, thespecial purpose port might be the Accelerated Graphics Port (AGP), usedto couple a high performance video card to the processing system 300. Inaddition to memory device 331 which may contain a buffer device of thepresent invention, any other data input device of FIG. 3 may alsoutilize a buffer device of the present invention including the CPU 301.

The storage controller 311 couples one or more storage devices 313, viaa storage bus 312, to the peripheral bus 310. For example, the storagecontroller 311 may be a SCSI controller and storage devices 313 may beSCSI discs. The I/O device 314 may be any sort of peripheral. Forexample, the I/O device 314 may be an local area network interface, suchas an Ethernet card. The secondary bus bridge may be used to interfaceadditional devices via another bus to the processing system. Forexample, the secondary bus bridge may be an universal serial port (USB)controller used to couple USB devices 317 via to the processing system300. The multimedia processor 318 may be a sound card, a video capturecard, or any other type of media interface, which may also be coupled toone additional devices such as speakers 319. The legacy device interface320 is used to couple legacy devices, for example, older styledkeyboards and mice, to the processing system 300. In addition to memorydevice 331 which may contain a buffer device of the invention, any otherdata input device of FIG. 3 may also utilize a buffer device of theinvention, including a CPU 301.

The processing system 300 illustrated in FIG. 3 is only an exemplaryprocessing system with which the invention may be used. While FIG. 3illustrates a processing architecture especially suitable for a generalpurpose computer, such as a personal computer or a workstation, itshould be recognized that well known modifications can be made toconfigure the processing system 300 to become more suitable for use in avariety of applications. For example, many electronic devices whichrequire processing may be implemented using a simpler architecture whichrelies on a CPU 301 coupled to memory components 308 and/or memorybuffer devices 304. These electronic devices may include, but are notlimited to audio/video processors and recorders, gaming consoles,digital television sets, wired or wireless telephones, navigationdevices (including system based on the global positioning system (GPS)and/or inertial navigation), and digital cameras and/or recorders. Themodifications may include, for example, elimination of unnecessarycomponents, addition of specialized devices or circuits, and/orintegration of a plurality of devices.

While the invention has been described and illustrated with reference tospecific exemplary embodiments, it should be understood that manymodifications and substitutions can be made without departing from thespirit and scope of the invention. Accordingly, the invention is not tobe considered as limited by the foregoing description but is onlylimited by the scope of the appended claims.

1-16. (canceled)
 17. A method of operating a wordline decoder,comprising: receiving and decoding wordline address information andsetting a latch associated with a wordline to a predetermined state whenan address of said wordline is decoded; supplying a voltage from avoltage pump to a first output inverter for turning on said wordline;providing a signal by said first output inverter in response to thestate of said latch being set to said predetermined state; driving asignal line with said signal provided by said first output inverter;driving said selected wordline with a second output inverter having aninput connected to an output of said first output inverter; and drivingsaid selected wordline with said second output inverter to a voltagebelow ground when turning off said wordline.
 18. The method of claim 17,wherein a size of a pair of cross-coupled transistors within said latchis chosen so as to minimize power consumption.
 19. The method of claim17, further comprising: shifting output levels of said first outputinverter between a pumped voltage a potential lower than ground.
 20. Themethod of claim 17, further comprising: supplying a voltage sink to saidsecond output inverter for turning off said wordline.
 21. The method ofclaim 17, wherein a size of a pair of complementary transistors withinsaid output inverter is chosen to have maximum switching speed whilestill minimizing power consumption.
 22. The method of claim 17, furthercomprising: supplying a voltage by a voltage pump to said first outputinverter.
 23. A method of operating a wordline decoder, comprising:receiving and decoding wordline address information and setting a latchassociated with a wordline to a predetermined state when an address ofsaid wordline is decoded; providing a signal by said first outputinverter in response to the state of said latch being set to saidpredetermined state; driving a signal line with said signal provided bysaid first output inverter; and shifting output levels of said firstoutput inverter between a pumped voltage a potential lower than ground.24. The method of claim further comprising: driving said selectedwordline with a second output inverter having an input connected to anoutput of said first output inverter; and driving said selected wordlinewith said second output inverter to a voltage below ground when turningoff said wordline.
 25. The method of claim 23, wherein a size of a pairof cross-coupled transistors within said latch is chosen so as tominimize power consumption.
 26. The method of claim 23, furthercomprising: supplying a voltage from a voltage pump to a first outputinverter for turning on said wordline.
 27. The method of claim 23,further comprising: supplying a voltage sink to said second outputinverter for turning off said wordline.
 28. The method of claim 23,wherein a size of a pair of complementary transistors within said outputinverter is chosen to have maximum switching speed while stillminimizing power consumption.
 29. The method of claim 23, furthercomprising: supplying a voltage by a voltage pump to said first outputinverter.